tailieunhanh - Chapter 040. Diarrhea and Constipation (Part 3)

Colonic Motility and Tone The small intestinal MMC only rarely continues into the colon. However, short duration or phasic contractions mix colonic contents, and high-amplitude (75 mmHg) propagated contractions (HAPCs) are sometimes associated with mass movements through the colon and normally occur approximately five times per day, usually on awakening in the morning and postprandially. Increased frequency of HAPCs may result in diarrhea or urgency. The predominant phasic contractions in the colon are irregular and nonpropagated and serve a "mixing" function. . | Chapter 040. Diarrhea and Constipation Part 3 Colonic Motility and Tone The small intestinal MMC only rarely continues into the colon. However short duration or phasic contractions mix colonic contents and high-amplitude 75 mmHg propagated contractions HAPCs are sometimes associated with mass movements through the colon and normally occur approximately five times per day usually on awakening in the morning and postprandially. Increased frequency of HAPCs may result in diarrhea or urgency. The predominant phasic contractions in the colon are irregular and nonpropagated and serve a mixing function. Colonic tone refers to the background contractility upon which phasic contractile activity typically contractions lasting 15 s is superimposed. It is an important cofactor in the colon s capacitance volume accommodation and sensation. Colonic Motility after Meal Ingestion After meal ingestion colonic phasic and tonic contractility increase for a period of 2 h. The initial phase 10 min is mediated by the vagus nerve in response to mechanical distention of the stomach. The subsequent response of the colon requires caloric stimulation and is mediated at least in part by hormones . gastrin and serotonin. Defecation Tonic contraction of the puborectalis muscle which forms a sling around the rectoanal junction is important to maintain continence during defecation sacral parasympathetic nerves relax this muscle facilitating the straightening of the rectoanal angle Fig. 40-1 . Distention of the rectum results in transient relaxation of the internal anal sphincter via intrinsic and reflex sympathetic innervation. As sigmoid and rectal contractions increase the pressure within the rectum the rectosigmoid angle opens by 15 . Voluntary relaxation of the external anal sphincter striated muscle innervated by the pudendal nerve in response to the sensation produced by distention permits the evacuation of feces this evacuation process can be augmented by an increase in intraabdominal .